Chapter: Software Engineering - Software Design

Design process

Software design is the process by which an agent creates a specification of a software artifact, intended to accomplish goals, using a set of primitive components and subject to constraints.

Design process

Software design is the process by which anagentcreates a
specification of asoftware
artifact, intendedto accomplish
goals, using a set of primitive components and subject to constraints.
Software design may refer to either "all the activities involved in
conceptualizing, framing, implementing, commissioning, and ultimately modifying
complex systems" or "the activity following requirements
specification and before programming, as ... [in] a stylized software
engineering process.

Software design usually
involves problem solving and planning a software solution. This includes
both low-level component and algorithm design and high-level, architecture
design.

Software design is both a
process and a model. The design process is a sequence of steps that enable the

designer to describe all
aspects of the software to be built. It is important to note, however, that the
design process is not simply a cookbook. Creative skill, past experience, a
sense of what makes “good” software,

and an overall commitment to
quality are critical success factors for a competent design. The design model
is the equivalent of an architect’s plans for a house. It begins by
representing the totality of the thing to be built (e.g., a three-dimensional
rendering of the house) and slowly refines the thing to provide guidance for
constructing each detail (e.g., the plumbing layout). Similarly, the design
model that is created for software provides a variety of different views of the
computer software. Basic design principles enable the software engineer to
navigate the design process. Davis [DAV95] suggests a set of principles for
software design, which have been adapted and extended in the following list:

·The design process should not
suffer from “tunnel vision.” A good designer should consideralternative approaches, judging each
based on the requirements of the problem, the resources available to do the
job.

·The design should be
traceable to the analysis model. Because a single element of the designmodel often traces to multiple
requirements, it is necessary to have a means for tracking how requirements
have been satisfied by the design model.

·The design should not
reinvent the wheel. Systems are constructed using a set of designpatterns, many of which have likely been encountered before. These
patterns should always be chosen as an alternative to reinvention. Time is
short and resources are limited! Design time should be invested in representing
truly new ideas and integrating those patterns that already exist.

·The design should “minimize
the intellectual distance” between the software and the problem as it exists in
the real world. That is, the structure of the software design should(whenever possible) mimic the
structure of the problem domain.

·The design should exhibit
uniformity and integration. A design is uniform if it appears thatone person developed the entire thing.
Rules of style and format should be defined for a design team before design
work begins. A design is integrated if care is taken in defining interfaces
between design components.

·The design should be
structured to accommodate change. The design concepts discussed inthe next section enable a design to
achieve this principle.

·The design should be
structured to degrade gently, even when aberrant data, events, or operating
conditions are encountered. Well-designed
software should never “bomb.” It shouldbe
designed to accommodate unusual circumstances, and if it must terminate
processing, do so in a graceful manner.

·Design is not coding, coding
is not design. Even when detailed procedural designs are createdfor program components, the level of
abstraction of the design model is higher than source code. The only design
decisions made at the coding level address the small implementation details
that enable the procedural design to be coded.

·The design should be assessed
for quality as it is being created, not after the fact. A varietyof design concepts and design measures are available to assist the
designer in assessing quality.

·The design should be reviewed
to minimize conceptual (semantic) errors. There is sometimesa tendency to focus on minutiae when the design is reviewed,
missing the forest for the trees. A design team should ensure that major
conceptual elements of the design (omissions, ambiguity, inconsistency) have
been addressed before worrying about the syntax of the design model.